It could have been caused by anything by seeing as he held on, likely something to do with food.
Most deffensive bites are just a grab and release while every bite from a boa I have got involving food or just the smell have resulted in constriction and holding on to some degree.

My main question for you is when was he fed last before he bit you and were you handling or even in the some area with any mammals before this? I know of a guy that got "attacked" because he was previously handling his cat as well as a similar story related to me because I was playing with our conure prior to getting Ruby out.

I am going to say it wasn't deffensive from my own observations but I am in terms an expert

As for your brother, you should be able to explain to him that this is a wild animal and it infact still has its natural instincts.